1. Field of the Invention
The present invention generally relates to a filter circuit for use in radio communication equipment, such as portable mobile radio equipment, a mobile telephone system, a cordless telephone, or the like, and particularly relates to a ladder-type piezo-electric filter to be used in a high-frequency region not lower than 1 MHz.
2. Discussion of the Related Art
Conventionally, ladder-type electric filters for radio communication equipment have been designed to operate with a center frequency of 455 kHz. As shown in FIG. 12, such a filter typically has a configuration in which piezo-resonators S, with resonance frequency at 455 kHz, and piezo-resonators P, with antiresonance frequency at 455 kHz, are inserted into series and parallel branches of a ladder-type circuit. Each piezo-resonator has a pair of electrodes placed respectively on the front and back surfaces of a square piezo-electric ceramic element. This filter, however, has the disadvantage that its size is large, as each of the piezo-resonators occupies an area of about 5 mm square.
To eliminate this disadvantage, a filter has been proposed which uses piezo-resonators that generate longitudinal harmonic oscillation. Each piezo-resonator has a pair of opposite electrodes placed respectively on the front and back surfaces of a rectangular piezo-electric ceramic element. Because its longitudinal or transverse size is at most 7/10 that of the square piezo-resonator, this filter has the advantages of a smaller size, a reduction in the amount of piezo-electric material used, and a lower production cost.
For the radio communication equipment of mobile telephone systems, there is a demand for a ladder-type piezo-electric filter of decreased size and capability of operating in a high-frequency region of approximately 1.7 MHz-2 MHz. Such a filter requires resonators of small dimension in both the longitudinal and transverse directions and, from the viewpoints of production and handling, is difficult to realize.